Flotation plants are used to an ever-increasing extent in the purification of water, both drinking water and sewage water, with the intention of increasing the rate at which the water flows through the purification plant and therewith also enable the size of the necessary tanks and thus the purification plant has a whole to be reduced. With regard to drinking water, the requirements of the end product are essentially unitary and the construction of a purification plant is essentially determined by the nature of the water source or procurement. When the water source is a lake or river, coarse particles are normally separated from the water in a screening chamber, whereafter the water is purified chemically and biologically by adding thereto precipitation and flocculating agents in a flocculating chamber. The precipitated particles are then separated from the water, by allowing the particles to settle to the bottom of a sedimentation tank gravitationally. Different types of particles will have different densities and therewith different settling rates. In order for particles of low densities, "light particles", to have time to settle as the water passes through a sedimentation tank, the tank must be given a large surface area so to provide the long period of time taken for such light particles to settle.
Particle separation times can be greatly reduced by flotation processes, in which there are added to the water microscopic air bubbles to which particles or particle flocs in the water adhere. These bubbles also contribute in forming particle flocs and in maintaining floc coherency. Air bubbles rise quickly to the surface of the water, carrying with them the adhered particles and particle flocs, these flocs forming a stable coating of slime on the surface of the water, which can be removed with the aid of scrapers or by intermittently raising the water level in the flotation tank so that the slime coating will flow into a slime chute provided at one edge of the tank.
The microscopic air bubbles used in flotation processes have a diameter of 30-80 .mu.m and cannot be produced by simply injecting air directly into water, for instance. Microscopic air bubbles intended for drinking water flotation processes are normally produced by releasing pressurized air into clean water from a pressure vessel. The highest possible degree of saturation is sought for in this regard. This water, normally referred to as dispersion water, is conducted to an inlet for water that has undergone a flocculation process, so-called flocculated water, and which is situated at the bottom of the flotation tank, while maintaining the high pressure of said water, and is there delivered to the flocculated water through separate nozzles or jets which are constructed to generate an instantaneous decrease in pressure to a level at which the air dissolved in the water is released in the form of microscopic bubbles and forms a dispersion of air in the water.
Those pressure vessels used to produce highly pressurized dispersion water that has been saturated with air to the greatest possible extent are normally kept half-filled with water with the aid of control equipment and associated water level sensing means. Compressed air is delivered to the air-filled space above the water surface and the water is often delivered through a nozzle mounted in a vessel side wall in the air-filled space. This water is injected onto the opposing vessel side wall such that the water jet will disintegrate to some extent as it impinges thereon.
The flotation method provides a considerably shorter water purification time, and the shortened through-passage time enables purification plants to be made smaller without impairing their earlier capacity. This reduction in the space requirement of purification plants opens new avenues of use However, the method does not only carry with it certain advantages. Thus, the method also makes the purification method more expensive, because water that has already been purified is recycled and used to produce dispersion water In order for the method to be made economical, refined methods are required to maintain the Consumption of dispersion water at a low level, pertly by endeavouring to increase the degree of air saturation in the dispersion water, and partly by improving dispersion as the dispersion water flows in to the flotation tank.
As previously mentioned, the dispersion water is introduced into the flotation tank through a number of nozzles, each of which includes a throttle valve which achieves the instantaneous decrease in the pressure of the dispersion water required to release the air in said water and therewith form microscopic bubbles The nozzles are often mounted in a rake-like configuration on one side of a distributor pipe or conduit means that has a length of several meters and to which the water is delivered through a feeder pipe mounted midway along the distributor pipe, essentially at right angles to the nozzles. With regard to the rake-like configuration of the nozzles, the feeder pipe can be likened to the hole in which the rake handle is fitted A "rake" can include twenty-five nozzles screwed into sleeves welded on the distribution pipe. Distribution pipes complete with nozzles are relatively expensive, and when the nozzles are made of plastic, the nozzles are easily damaged, for instance when cleaning the tanks.